Composite semiconductor light source pumped by a spontaneous light emitter

ABSTRACT

A composite light source includes a substrate having a top surface, and a first vertical light source formed in the substrate. The first light source includes least a lower mirror, a first active region above the lower mirror, wherein the first active region has a thickness sufficient when electrically pumped to emit predominantly a spontaneous vertical emission from the first active region towards the top surface having an angular range of at least (≧) 30°. A second light source is formed in the substrate above the first active region including a second active region. The spontaneous vertical emission is at a first wavelength λ 1  that optically drives said second active region to provide an emission at a second wavelength λ 2 , wherein λ 2 &gt;λ 1 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) application which claims the benefit of application Ser. No. 13/352,050 entitled “SEMICONDUCTOR LIGHT SOURCES INCLUDING SELECTIVE DIFFUSION FOR OPTICAL AND ELECTRICAL CONFINEMENT” filed on Jan. 17, 2012, now U.S. Pat. No. 8,774,246, which claims the benefit of Provisional Application Ser. No. 61/432,938 entitled “SEMICONDUCTOR LIGHT SOURCES INCLUDING DEPLETED HETEROSTRUCTURES FOR OPTICAL AND ELECTRICAL CONFINEMENT”, filed Jan. 14, 2011, which are both herein incorporated by reference in their entireties.

GOVERNMENT INTERESTS

This invention was made with Government support under Air Force Research Laboratory (AFRL) of the Department of Defense (DOD) Phase II SBIR Contract No. FA8650-08-C-1418. The United States Government has certain rights in this invention.

FIELD

Disclosed embodiments relate to composite semiconductor light sources that include a first light source that optically pumps a second light source which provides an emission for the composite light source.

BACKGROUND

Semiconductor vertical cavity devices operate by emitting light in the normal direction to an epitaxial growth surface. In such a device a partial or complete cavity is formed on the same semiconductor substrate that includes the active light emitter. Semiconductor vertical cavity devices include vertical-cavity surface-emitting laser (VCSEL) diodes and resonant cavity light emitting diodes (RCLEDs).

A VCSEL is a laser resonator that includes two mirrors which are typically distributed Bragg reflector (DBR) mirrors which have layers with interfaces oriented substantially parallel to the die or wafer surface with an active region including of one or more bulk layers, quantum wells, quantum wires, or quantum dots for the laser light generation in between. The planar DBR-mirrors comprise layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivities that may be above 99%.

A laser device typically has a threshold current that is determined by the cavity and gain design. For current levels above this threshold current the device operates as a laser. For currents below this threshold current the device operates in a spontaneous regime. It is also possible a device that emits spontaneous emission is unable to operate as a laser. When the current is above a lasing threshold, the laser generates heat in its light emission process since its quasi-Fermi energy separation for electrons and holes exceeds its emitted photon energy.

A laser also generates some heat when electrons and holes absorb some of the laser light but remain respectively in the conduction band or valence band, known as free carrier absorption. In this case, some of the laser light is absorbed, and this internal absorption also reduces the laser light output. A laser also generates some heat if electrical currents are used to transport electrons and holes to an active region with an external electrical bias. These voltage losses that come from the electron and hole charge transport and the optical absorption that comes from the electron and hole free carrier absorption decrease the laser's efficiency and increase its self-heating. The optical absorption increases the laser's threshold which further decreases its efficiency.

Because the light is most efficiently extracted from a vertical cavity diode's surface in the same region in which electrical current is injected into its active area, a particular problem in semiconductor vertical cavity emitting diodes is the efficient lateral confinement of their electrical current to the same region that confines the optical mode. Known approaches to this problem include the standard intra-cavity oxide approach that suffers from mechanical strain, increases in the vertical cavity diode's drive voltage, and blocking heat flow inside the vertical cavity diode.

Some semiconductor light sources are composite light sources which include an electrically pumped laser within a resonant cavity between a pair of mirrors that optically pumps another laser. The beam from the pump laser is a well directed (i.e., low angular spread) beam because of its cavity and emitter properties from the design of the laser. For this type of composite light source, the composite light source includes a cavity and gain design for the pump laser that brings the pump laser to threshold at a current drive that is less than the threshold of the laser to be pumped. This type of pump laser in a composite light source typically delivers a pump beam of light that has low angular spread due to stimulated emission.

SUMMARY

Disclosed embodiments include composite semiconductor light sources including a first vertical light source acting as a pump for a second light source, where the first light source has an active emitting region having a thickness sufficient (typically at least 0.1 μm to about 5 μm) so that the emission is predominantly a spontaneous emission defined herein to be at least 50.1% of the light emitted, with the remainder of the emitted light being light emitted by stimulated emission, with the spontaneous emission % typically being at least 99%. Depending on the needed pump power, thinner active emitting regions such as quantum wells or multiple quantum wells (which may be planar, quantum wire like, or quantum dot like) may also be used. The first light source can comprise a light emitting diode (LED).

As used herein and commonly understood by those having ordinary skill in the art, stimulated emission is an emission process by which an atomic electron (or an excited molecular state, or a conduction band electron of a semiconductor) interacts with an electromagnetic wave of a certain frequency to drop to a lower energy level, transferring its energy to the field of the electromagnetic wave. The new photon created in this manner has the same phase, frequency, polarization, and direction of travel as the photons of the incident wave. This is in contrast to spontaneous emission that Applicants use as the pump light source which occurs without regard to the ambient electromagnetic field, and comprises an emission process by which a material is in an excited state with energy E₂ and decays spontaneously into an energy state having energy E₁, so that a photon having a certain frequency can be released (the energy of the emitted photon usually not exceeding the difference between the two energy states (E₂−E₁).

For a conventional laser, such as a vertical-cavity surface-emitting laser (VCSEL) operating above its lasing threshold can have only a few percent (e.g., 5%) of its emission due to spontaneous emission, with the remainder of the emission being stimulated emission. When below threshold a VCSEL is typically very inefficient. This is because the spontaneous emission has a much wider radiation pattern such that only a small fraction can escape the laser cavity. Because of this spontaneous emission is recognized to typically be an inefficient means to optically pump. However it is recognized herein when placed in composite cavities or similar close coupling, spontaneous emission with a wide radiation pattern trapped in the semiconductor can also be used to efficiently optically pump another active light source. In this case the inability for light to escape the semiconductor is recognized to be beneficial for efficiency. For this to occur the lasing threshold of the pump source (e.g., LED) for lasing should be greater than that of the active source it is pumping. Under these conditions the pump source (e.g., LED) avoids stimulated emission and produces the wide radiation pattern that can be trapped in the cavity.

The differences between conventional optically pumping with a laser and disclosed optically pumping with a spontaneous emitter (e.g., with an LED) is that the spontaneous emitter has different emitter and cavity requirements for lasing. Since thick bulk emitting layers of the LED can provide more power in spontaneous emission, they have advantages in pumping at higher power. In this case, if inverted in its electron-hole population, the spontaneous emitter (e.g., LED) would likely produce a great deal of stimulated emission that may not couple to the gain of the laser to be pumped. For example lasing in the optical pump could produce stimulated modes that couple less than 1% or so of their light into the laser to be pumped. In this case the spontaneous emitter (e.g., LED) can be maintained to have a wide angular range (such as 30 to 45 degrees, up to close to 90 degrees) and produces a wide angle emission to pump the laser into laser operation. Accordingly, disclosed composite light sources produce a different optical pump pattern over a wide angular range, that results directly from the active emitter.

Some embodiments include light sources including a depleted heterojunction current blocking region (DHCBR) within an outer current blocking region of at least one of the upper mirror, lower mirror, and the active region. In this embodiment, the conducting channel is within the inner mode confinement region that physically contacts and is framed by the DHCBR, wherein the DHCBR functions to force current flow into the conducting channel during operation of the light source. The DHCBR includes a depleted heterojunction, which inherently includes a depletion region, as well the depleted heterojunction to provide the current blocking. As known in the art and used herein a “heterojunction” is the interface between two layers or regions of dissimilar crystalline semiconductors, where the respective semiconducting materials have unequal band gaps.

A DHCBR is defined herein to comprise first and second materials, where the respective materials are different materials (e.g., GaAs and AlGaAs) interfaced with one another to form a heterojunction. The doping in the first and second materials may be the same, or the doping in the second material may be less than the doping in the first material. The first material being interfaced to the second material in the heterojunction depletes the mobile charge carriers of the second material to form a highly resistive region in the second material, so that the second material has no more than about 1×10¹⁷ cm⁻³ mobile charge carriers due to dopants. As such, the second material in the heterojunction is referred to herein as a depletable second material since it depletes in mobile charge concentration, such as from a concentration of about 10¹⁷ cm⁻³ when in isolation, to a concentration of about 10⁸ cm⁻³ to about 10¹⁵ cm⁻³ when in the heterojunction interfaced to the first material.

A range of heterojunction materials and doping concentrations can be used to produce disclosed DHCBRs. An important DHCBR aspect is that the DHCBR possesses an electrical resistance that is sufficiently high to prevent leakage currents around an active region. It is useful to characterize the electrical resistance in terms of a contact resistance normalized to a blocking area, in units of ohms·cm². In some embodiments it is desirable that the DHCBR also have low capacitance to enable high speed modulation of the light source.

By disclosed composite light source embodiments using predominantly spontaneous emission to optically drive the second light source, the spontaneous emitting pump light source can operate with a drive voltage less than the corresponding photon energy. Accordingly, disclosed semiconductor light sources can remove heat generating electrons and holes from the active region of the second light source that originate from the presence of dopants, by eliminating or reducing the concentration of its dopants in its active region. Moreover, by using a predominantly spontaneous emission for pumping, off-normal axis optical pump energy due to the spontaneous emission can be readily absorbed in the second light source to provide the emission for the compound light source. In addition the pump energy obtained from spontaneous emission can be generated with very high overall efficiency. In principle the efficiency of the spontaneous emitter (e.g., LED) can exceed unity and enable it to operate as both an optical pump and a heat pump. The heat pump created by the active emission can absorb heat from current flow through electrical contacting and heat given off by the optically pumped laser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional diagram of a semiconductor vertical cavity diode light source that includes a selective diffusion which provides a conducting channel through an otherwise current blocking layer provided by a DHCBR which provides both optical and electrical confinement in an upper mirror region of the laser cavity, according to an example embodiment;

FIGS. 1B and 1C are band diagrams depicting the shape of energy bands of a DHCBR lateral to the conducting channel of the light source shown in FIG. 1A under zero bias and a forward bias, respectively;

FIG. 2 is a schematic cross sectional diagram of a semiconductor vertical cavity diode light source that includes a selective diffusion that forms a reversed biased n-p junction in an outer current blocking layer which provides electrical and optical confinement through an active layer and into alternating layers of different material compositions in the upper or lower mirrors, according to an example embodiment.

FIGS. 3A and 3B are a side view and a cavity end view, respectively, of an example composite light source that integrates a vertical cavity light source with a lateral cavity light source (mirrors not shown) that provides edge emission by embedding the lateral cavity layers inside the vertical light source, with an n-electrode on top of the substrate and a p-electrode on the bottom side of the lower mirror, according to an example embodiment.

FIG. 4 shows an example composite light source that integrates an electrically pumped vertical cavity light source embodied as a spontaneous emitter with a vertical cavity light source that provides surface emission, with an n-electrode just above the active region and a p-electrode on the bottom side of the lower mirror, according to an example embodiment.

FIG. 5 shows an example composite light source that integrates an electrically pumped vertical cavity light source embodied as a spontaneous emitter with a vertical cavity light source that provides surface emission, where the top electrode includes a center aperture to allow light emission therethrough, according to an example embodiment.

FIG. 6 shows measured data for an example InGaAs quantum well p-n diode laser including a DHCBR and p-diffusion conducting channel based on the arrangement shown in FIG. 1A, according to an example embodiment of the current density vs. voltage, for a variety of geometries from 15 μm to 100 μm.

FIGS. 7A and 7B show spontaneous emission spectra and lasing spectra for the laser described relative to FIG. 6 pumped by an LED which provides predominantly spontaneous emission.

DETAILED DESCRIPTION

Disclosed embodiments in this Disclosure are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the disclosed embodiments. Several aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the disclosed embodiments.

One having ordinary skill in the relevant art, however, will readily recognize that the subject matter disclosed herein can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring structures or operations that are not well-known. This Disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of this Disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5.

Disclosed embodiments include semiconductor light sources including diode lasers and arrays of such lasers based on selective (non-blanket) buried acceptor (p-type) diffusions or selective donor (n-type) diffusions that simultaneously provide both the electrical injection path into the laser and optical mode confinement. Associated with the diffusion are particular material properties that result from how a donor or acceptor impurity atomic concentration profile exists across heterointerfaces. A dopant impurity diffusion creates unique material profiles of the impurity distribution through the heterojunctions due to the diffusion process, and these produce structures in the final device which can be determined using different characterization techniques, such as secondary ion mass spectroscopy (SIMS) or others.

An optical mode confining phase shift may also be included in an otherwise planar resonant cavity. In one embodiment each acceptor or donor diffusion provides a conducting channel that defines the mode confinement area of a small resonant cavity light emitting diode, or a VCSEL. Disclosed arrangements can reduce the operating current or voltage and control the optical mode for improved sensors, data communication, high power light sources for optical pumping or materials processing, as well as other applications.

In some light source embodiments a DHCBR is formed within an outer current blocking region of at least one of an upper mirror, a lower mirror, and the first active region that frames a conducting channel of the light source. The DHCBR forces current flow into the inner mode confinement region during operation of disclosed light sources. A selective dopant diffusion can also be performed in the inner mode confinement region to form a conducting channel laterally inside the DHCBR to further reduce electrical resistance into inner mode confinement region. The dopant diffusion can be either an acceptor diffusion or a donor diffusion.

In other embodiments, an acceptor diffusion can be performed through an active layer into a region vertically adjoining the active layer that contains differing material compositions containing donor dopants. These differing material compositions can be such that the acceptor diffusion incorporates to a greater degree into one of the material compositions to change its electrical conductivity by producing an acceptor concentration that exceeds the donor concentration, while incorporation into the differing material composition to a lesser degree to maintain an acceptor concentration that is less than the donor concentration. In this manner a conductivity change can be obtained between the differing material compositions vertically adjoining the active layer, and thus impede the current flow. Thus the acceptor diffusion can form a buried acceptor diffusion, in which the acceptor passes through one material composition to incorporate mainly in a second and differing material composition. It is also possible that the acceptor concentration exceeds the donor concentration in a material with lower incorporation, but forms a DHCBR after diffusion due to heterojunctions and the respective material compositions.

When an acceptor diffusion is performed through an active layer it can also modify the active layer composition, causing intermixing between the active layer and surrounding materials. This intermixing with surrounding materials can further provide mode and electrical confinement, by changing the active material composition and thus its emission energy and refractive index.

In some embodiments, mesas are included. Mesas are known in the art as being structures that provide a step height which can improve current confinement, mode confinement (e.g., to provide single mode operation), and also reduce the threshold current for lasing. Fabrication methods are also disclosed herein that include using material beneath mesas initially not doped with dopant impurities to create a conductivity type different than the mesa and the semiconductor material above the mesa. Diffusion of dopant impurities from the mesa into the semiconductor material below the mesa can convert the semiconductor material immediately below to the same type as the mesa, while regions outside and below the mesa can remain substantially free of dopants. In this way a DHCBR can be formed outside the mesa to provide a high resistance region while the mesa region itself and the layer immediately below the mesa can provide low electrical resistance. Thus, electrical current may be forced to flow mainly through the mesa in the inner mode confinement region, so that efficient electrical injection is made into the same region that confines the optical mode.

FIG. 1A is a schematic cross sectional diagram of a semiconductor vertical cavity diode light source (light source) 100 that includes a selective diffusion which provides a conducting channel 131 through an otherwise current blocking layer provided by a DHCBR 126(a) in an upper mirror shown as upper DBR 110 of the laser cavity 145, according to an example embodiment. In this embodiment, the conducting channel 131 is within the inner mode confinement region that physically contacts and is framed by the DHCBR 126(a), wherein the DHCBR 126(a) functions to force current flow into the conducting channel 131 during operation of the light source 100.

Upper DBR 110 may also be replaced with other mirror schemes, such as combined with dielectric or metal layers. It may also include an optical coupling element. Although light source 100 is shown in FIG. 1A as being top emitting, disclosed light sources can be also be side emitting or bottom emitting. In the embodiment described relative to FIG. 1A, the substrate 105 can be an n-GaAs or an n+ GaAs substrate. Other substrates and epitaxial materials can be used, such as InP, GaN, sapphire, GaP, InAs, Si, Ge, which are all commonly used to form semiconductor light emitters.

Although a substrate 105 is shown in FIG. 1A and other FIGs. herein, in some process flows the substrate 105 is removed at or near the end of processing, so that the light source 100 only includes the epitaxial layers that form the light source, with no supporting substrate. In this process flow the light source is generally transferred to another substrate. For example, a substrate-less light source may be transferred to a silicon substrate for integration with silicon-based electronics, so that the epitaxial film of the laser can be grown on its matching substrate (e.g., GaAs), since it may not be able to be grown on a silicon substrate due to epitaxial incompatibility. It may also be inserted into a display.

Light source 100 includes an inner mode confinement region 133 and an outer current blocking region 132. As described below, a disclosed selective diffusion forms the conducting channel 131 shown as part of the inner mode confinement region 133.

The upper DBR 110 includes an electrode 111, such as an acceptor (p) doped p+ contact. Light source 100 also includes a lower mirror shown as a lower DBR 120, such as donor (n) doped mirror having an electrode 121 such as an n-electrode having an N+ contact. The DBRs 110, 120 each generally include 10 to 30 periods, such as 18-25 periods for the lower DBR 120 and 15 to 20 periods for the upper DBR 110. In one specific embodiment, upper DBR 110 can comprise p-AlGaAs/GaAs stacks, while the lower DBR 120 can comprise n-AlGaAs/GaAs stacks. The light source 100 is formed in at least two epitaxial growth steps. Lower DBR 120 can also be replaced with output coupling layers, or a mirror approaching zero reflectivity.

The upper DBR 110 and lower DBR 120 can each comprise fully semiconductor DBRs, or can comprise partial DBRs with added metal or dielectric layers that provide added reflectivity. In the case of added metal or dielectric layers the layer thicknesses can be chosen to obtain the needed interference in the emitted light such that the desired reflectivity is achieved. These thicknesses depend on the specific material parameters as known in the art.

An active region 115 is positioned between the respective DBR's 110 and 120. The active region 115 can comprise at least one bulk layer, quantum well (QW) layer, quantum wire layer, or quantum dot layer. An example QW layer comprises InGaAs formed between adjoining GaAs layers. More than one layer may make up active region 115, such as multiple QW layers, bulk layers, quantum dot layers, quantum wire layers, and combinations thereof.

One of the layer stack heterojunctions (i.e., junctions between two materials with different band gaps, e.g., AlGaAs/GaAs stacks) in the upper DBR 110 is shown as heterojunction 126. Heterojunction 126 includes DHCBR 126(a) within its outer current blocking region 132 and a conducting channel 131 in its inner mode confinement region 133. DHCBR 126(a) provides both optical mode and electrical confinement.

Conducting channel 131 can include acceptor doping in the range from about 5×10¹⁷ cm⁻³ to about 1×10²⁰ cm⁻³, while the DHCBR 126(a) lateral to the conducting channel 131 has a doping level on its more lightly doped side that may be the same, or be significantly below, that of conducting channel 131 to provide lateral electrical confinement, such as a maximum acceptor doping of 1×10¹⁷ cm⁻³, maximum doping of 5×10¹⁶ cm⁻³, maximum doping of 1×10¹⁶ cm⁻³, such as a maximum doping of 1×10¹⁵ cm⁻³, including an even lower doping level when intentionally not doped during fabrication. For example, intentional doping may be eliminated from the more lightly doped side of the DHCBR 126(a), so that it contains only background impurity concentrations due to the epitaxial growth, often less than ˜10¹⁵ cm⁻³, such as doping in the range from 1×10¹⁰ cm⁻³ to 1×10¹⁵ cm⁻³. The depletable material side of DHCBR 126(a) has a voltage dependent depletion region thickness in the vertical direction shown as Δt in FIG. 1A.

The current blocking ability of the DHCBR 126(a) can be described by the contact resistance it creates, given in the units of ohm·cm² (′Ω·cm²). This contact resistance is given approximately by the equation below in terms of the depletion layer thickness, hole concentration in the depleted layer, p_(DHCBR), and hole mobility, μ_(h), as

$\begin{matrix} {\rho_{DHCBR} = \frac{\Delta\; t}{q\;\mu_{h}p_{DHCBR}}} & (1) \end{matrix}$ where q is the electronic charge. The hole concentration in the depleted layer is set by the valence band discontinuity as well as the doping levels. As an example, a DHCBR 126(a) formed from undoped AlAs with Δt=850 Å clad by p-type GaAs with the GaAs acceptor doped to 10¹⁸ cm⁻³, could yield a hole concentration in the AlAs as low as 2×10⁹ cm⁻³. The corresponding contact resistance of the DHCBR 126(a) will be approximately 200′Ω·cm². Even a 1 V drop across the DHCBR would produce only ˜0.005 A/cm² leakage current density around the active device. A 250 μm×250 μm light source 100 embodied as a VCSEL chip would produce ˜3 μA of leakage current, which is considered to be negligible. Etching can also be used to further restrict the device area that includes the DHCBR 126(a), to achieve low capacitance or dense packing in arrays, and eliminate leakage currents through the DHCBR relative to the active region current in the conducting channel 131.

Lithography applied to inner mode confinement region 133 after an initial epitaxial growth step can be used to define selective doping in conducting channel 131, and thus very small size devices since the device area is generally only limited by the resolution of the lithography tool (e.g., ≦50 nm). This lithography step and semiconductor surface processing through etching can be used to remove some of the semiconductor material in outer current blocking regions 132 in a given layer (or layers) to enable the conducting channel 131 to be in the form of the mesa structure shown. The mesa structure can occur anywhere in the upper DBR 110.

To achieve the minimum threshold for a given device size however it can be an advantage if the conducting channel 131 is placed within two optical wavelengths (free space wavelength divided by the refractive index n_(f) in the crystal) distance to the active region 115, and for the mesa to be between 5 Å to 300 Å in height, with the optical wavelength relative to that in the semiconductor material. An acceptor-diffusion from the remaining semiconductor material in the conducting channel 131 not removed using lithography can then be performed, including in the epitaxial growth apparatus. If the diffusion is performed outside the growth chamber, after diffusion the semiconductor surface can then be cleaned and the wafer placed back in the semiconductor epitaxial growth apparatus to form subsequent epi layers to complete the upper DBR 110.

The diffusion from the remaining crystal region can be performed in the epitaxial growth apparatus. Thus, conducting channel 131 can provide a mesa including an acceptor diffusion level (or donor diffusion level) sufficient to provide the conducting channel 131 in inner mode confinement region 133 through an otherwise current blocking layer heterojunction 126.

The band diagrams shown in FIGS. 1B and 1C depict the DHCBR 126(a) shown in FIG. 1A under zero bias and a forward bias, respectively. The width of the depletion region Δt in DHCBR 126(a) can be seen to increase with a forward bias voltage applied (forward bias applied between electrodes 111 and 121 which serves to further block the current flow across the DHCBR 126(a) and thus direct the current to the conducting channel 131. If two heterojunctions are used, the depletion width may remain fixed with applied bias.

Although the conducting channel 131 is provided by an acceptor diffusion as described relative to FIG. 1A, as noted above, the conducting channel 131 can also be formed n doped by using an donor impurity (n-type) diffusion instead of acceptor impurity diffusion. In this arrangement the depleted heterojunction 126 is also embedded in n-type material to form the outer current blocking region 132.

Considering the contact resistance for the p-conducting channel 131 with thickness Δt, it can given approximately by:

$\begin{matrix} {\rho_{Channel} = \frac{\Delta\; t}{q\;\mu_{h}p_{Channel}}} & (2) \end{matrix}$ where now p_(channel) is the hole concentration in the conducting channel 131. Considering an acceptor diffusion, the hole concentration in the conducting channel may be 10¹⁷ cm⁻³. The contact resistance of the conducting channel could be less than 5×10⁻⁶′Ω·cm², more than 8 orders of magnitude less than for some DHCBR designs. Even for a high current density excitation of 10⁴ A/cm², the voltage drop over the conducting channel would be only ˜50 mV.

The conducting channel 131 however can also suffer increased contact resistance due to heterojunction effects. In addition, the hole concentration in the conducting channel 131 formed by acceptor diffusion will in general depend on the incorporation of acceptor impurity in the conducting channel materials. Therefore the particular material compositions making up the DHCBR 126(a) can be selected based on an objective to achieve both a current blocking characteristic in outer current blocking region 132 and a conductivity level in the conducting channel 131 for low voltage drive in inner mode confinement region 133. Higher hole concentrations can decrease the voltage drop over the conducting channel 131 to a few mV.

A significant feature of disclosed DHCBR 126(a) is that its improved temperature properties in the VCSEL can be used to make improved optical data links. The optical link generally includes a housing for electronic circuitry that receives data in electrical form and conditions the electrical signals to drive DHCBR comprising VCSELs that produce the optical data signals. This housing increases in temperature during operation of the optical link, which increases the temperature of operation of the VCSELs. The speed with which optical data can be transmitted by the link through an optical medium such as an optical fiber, waveguide, or free space, and received by photodiodes and additional electronic circuitry, that converts the optical data back to electrical data, may largely be set by the temperature performance and speed of the VCSELs.

Transceivers place the transmitting and receiving electronics and photonics components in a common housing, and allow both sending and receiving data by electronic circuitry, VCSELs, and photodiodes contained in a single housing. If the optical medium is optical fiber, and the fiber permanently fastened to the housing contains the interface electronics, VCSELs and photodiodes, the complete system can form an active optical cable. By using DHCBR comprising VCSELs, the optical links including active optical cable can be made to operate over a wider temperature range, and/or with higher speed, and with greater reliability. These advantages are important for applications, for example, in which large numbers of transceivers are used in close proximity and cause heating, consumer products based on active optical cables that may have small housings to conserve space, and optical data links used in space and satellite communications.

The thickness (Δt) of the DHCBR 126(a) can range from ˜10 nm to more than 0.2 μm. By increasing the thickness the capacitance of disclosed light sources, such as a VCSEL, can be reduced. At the same time the capacitance is reduced, the electrical and thermal resistances can remain low because of the all-epitaxial structure. An example of a low electrical resistance of a conducting channel 131 in a DHCBR 126(a) is with the data described below relative to FIG. 6.

The DHCBR 126(a) can also be used to make very small area light sources such as VCSELs and RCLEDs. This is because the conducting channel is formed lithographically, which also forms the lateral cavity of the device. Therefore the conducting channel 131 can be as small as 100 nm in diameter, or smaller based on the available lithographic tool. This provides the opportunity to make very small sized VCSELs and RCLEDs for dense integration into various systems, increasing the system's capabilities. The very small sized VCSELs can be used to transmit high speed optical data signals. The small sizes are particularly useful for integrating onto Si photonic chips, since the small sized VCSELs can couple their outputs to Si photonic waveguides. The small sized VCSELs are also important for improved sensors, since reducing their cavity size can improve their single mode properties.

Small RCLEDs or VCSELs are also useful for driving up-converters to produce red, green, and blue light sources. By making the RLCEDs and VCSELs with very small device sizes, a large number of devices can be simultaneously produced on as common substrate using low cost semiconductor processing. Disclosed devices can also be combined with epitaxial lift-off techniques, which could make the RCLEDs and VCSELs very low cost. The RCLEDs are especially attractive for this, since they can use low Al content in their n-type layers and mirrors to ease the lift off. This could make them useful for ultrathin displays, and/or curved displays. VCSELs may also be used this way.

The display can be comprised of a thin screen that can contain the up-converters and RCLEDs or VCSELs, and the electrical interconnection of the pixels. Matrix addressing can be used to interconnect the pixels, based on luminescence decay times of the up-converters. In addition, the display may include the electronic circuitry to receive a video display signal, and convert it to electrical driver signals for the RCLEDs or VCSELs to excite the up-converters and display an image.

In such a display the up-converters that emit the red, green, or blue light could be placed in their own small cavities. These can be metal coated, with Ag for example, for high output coupling and optical isolation. Other metals can also be used an may have advantages in cost or thin film material properties. These up-converter cavities may be 25 μm to 300 μm or more in lateral size, depending on the display and pixel resolution. The minimum size can be smaller than 25 μm, and set mainly by the up-converter particle size and RCLED or VCSEL size. The RCLEDs or VCSELs infrared light emission can be efficiently coupled to the up-converter cavity through a small aperture, so that once trapped, the infrared light emission is efficiently absorbed by the up-converters. By placing the up-converters in their own cavities, the pixels can be isolated. This can lead to very high quality red, green, and blue pixels in a display that may only be a few hundred microns thick, and/or with a curved screen.

If the conducting channel 131 and DHCBR 126(a) are defined in lateral size to be elliptical, or at least non cylindrical, the polarization of the light source such as a VCSEL may also be controlled. This polarization control can arise due to preferred reflectivity by the Bragg reflectors, favoring lower loss for one polarization vs. another. However, other mechanisms may also control the polarization. Controlling the polarization can be used to make improved sensors. This is because the cavity size can be made so small using the DHCBR and conducting channel that only one spatial lasing mode may be allowed with within the gain bandwidth of the active material. This gain bandwidth may include spectral shifts due to temperature changes, as well as electronic bandfilling. Elliptical or non cylindrical lateral confinement can then be used to control polarization.

The ability to make the cavity very small and control the polarization can lead to VCSEL sources capable of making improved sensors, especially those using Doppler sensing as well as atomic clocks. These sensors include housings that comprise the electronic circuitry and photonic components to direct light beams from the small lasers, and detect difference in the reflections of the light beams to perform sensing. Or, the VCSELs are used to excite rare earth atoms, by properly modulating the VCSEL output to create resonant optical drive conditions with the atoms. In these sensors the electronic circuitry and sensor performance depends on the single mode qualities of the VCSEL. Therefore these sensors can be improved by highly single mode, polarization controlled VCSELs that can be fabricated by reducing the size and controlling the shape of the conducting channel 131.

FIG. 2 is a schematic cross sectional diagram of a disclosed semiconductor vertical cavity diode light source (light source) 200 that provides optical and electrical confinement through a selective diffusion that forms a reversed biased n-p junction in an outer current blocking layer of outer current blocking region 132 through the active layer and into alternating layers of different material compositions in the upper or lower mirrors, according to an example embodiment. The different material compositions either allow or prevent the dopant from incorporating in the layer, and increase the voltage to limit current flow in the inner mode confinement region 133. The buried acceptor diffusion may be performed in a two-step growth process, where the buried acceptor diffusion is performed after an initial growth step and lithographic patterning, followed by a second growth step to complete the upper DBR 110.

The structure of light source 200 is analogous to light source 100 shown in FIG. 1A except the blocking structure provided by the selective diffusion comprises a selective acceptor diffusion into acceptor impurity diffusion region 231 and layer 120(a) which is applied to the outer current blocking region 132 in active region 115 and one or more periods above or below active region 115. Since lower DBR 120 is shown as an n-type DBR such as an n-type AlAs/GaAs DBR, acceptor impurity diffusion 231 being into an n-type DBR 120 forms a reverse biased (upon normal operation of light source 200) p-n junction blocking layer comprising an n-type layer (e.g., AlAs layer) as layer 120(a) that stays n-type after receiving the acceptor impurity-diffusion, while the other layer 120(b) being originally n-type layer (e.g., GaAs layer) 120(b) gets converted to p-type after receiving the acceptor impurity-diffusion.

In light source 200 the acceptor diffusion can be performed outside the inner mode confinement region 133 and diffused through the active region 115, n-type layer 120(a), and into one or more layers 120(b) to convert to p-type. Diffusion conditions can be fixed to obtain substantially different solubilities for the acceptor diffusion in layers 120(a) and 120(b) to form an n-p junction between these layers. For example, diffusion of Be (an acceptor) at temperatures less than about 615° C. can cause the Be to pass through layer 120(a), which in one embodiment may be AlAs, with little substitutional incorporation into the semiconductor crystal lattice, leaving layer 120(a) n-type. The Be substitutional incorporation into layer 120(b), which in one embodiment may be GaAs can at the same time be sufficient to convert this layer to p-type conductivity, forming a reverse bias p-n blocking junction to electrical current during normal operation of light source 200.

Layer 120(a), for example, can be formed from n-type AlAs or n-type InGaP doped with donor impurities at a concentration of 1×10¹⁷ cm⁻³ or higher, for which the transfer of Be (or other acceptor) impurity to substitutional lattice sites can be made to be minimal. Layer 120(b) can be formed from GaAs, which prior to the acceptor diffusion can be n-type but can be converted to p-type due to the Be diffusion and ease of the Be transferring to substitutional positions on the GaAs lattice. Other acceptor impurities, such as those found in the column II of the periodic table of elements, can also be used, such as Mg, Zn, or Cd. If intermixing is achieved in the active region 115 in the outer current blocking regions 132, additional mode confinement can be obtained through refractive index change due to the layer intermixing, and electrical confinement can be obtained due to an energy gap change resulting from the layer intermixing. The acceptor diffusion may be obtained from an implanted source, a source diffused from layers epitaxially grown on the surface and removed prior to a second epitaxial regrowth, or from the surface of the upper DBR 110.

It is also possible that the acceptor diffusion into the lower n-type mirror form one or more DHCBRs. This can occur when the diffusion and doping conditions are such that a very light hole concentration is formed in one layer that is depletable by the second layer. This can happen for example for n-type GaAs/AlAs layers, for which the n-type AlAs is doped lightly to n-type, and diffusion conditions drive most of the acceptor out of the AlAs except a small remaining fraction converting the AlAs to p-type. N-type GaAs/AlGaAs layers may also achieve this.

The mode confinement arrangements shown in FIGS. 1A and 2 can be combined to provide even greater electrical confinement to the inner mode confinement region 133. In the case the inner mode confinement region 133 includes a mesa, the electrical current can then be confined to the phase-shifting mesa even for a device with electrode placement that covers a much larger area than the phase-shifting, mode confining mesa. In this way the thickness of the additional layers outside the inner mode confinement region 133 to provide strong current blocking also can also function as optical alignment markers for additional lithography steps that follow the mesa and DBR formation.

FIGS. 3A and 3B show a side view and end view respectively, of an example composite light source 300 integrated on a single chip including a disclosed light source 100 together with a lateral light source 440 (mirrors not shown) having active region 441 that provides edge emission formed by embedding the lateral light source 440 within the vertical resonant cavity of light source 100 shown in FIG. 1A. As described above, a DHCBR 126(a) together with the conducting channel 131 provides mode confinement for the light source 100. Composite light source 300 includes a n-electrode 121 on the top of the substrate 105 and a p-electrode 111 on the bottom of the lower DBR 120 for biasing.

During operation of the composite light source 300, vertical spontaneous emission generated by the light source 100 in active region 115 at photon energy hν₁ is absorbed in the waveguide and/or active region 441 of lateral light source 440 to provide optical pumping to generate laser light at photon energy hν₂ which emits for the side of the active region 441. The condition for absorption of vertical cavity light is that hν₂<hν₁. In addition it is recognized to be an advantage to also have absorption in adjoining-layers to the active region (the waveguide) that can then transport their carriers to the active region 441 of lateral light source 440. This waveguide absorption occurs when the adjoining layers around the active region 441 have energy gaps less than the emission wavelength of the spontaneous optical pump.

As described above, spontaneous emission from a light source such as a LED and stimulated emission from a semiconductor laser or super luminescent diode are far different processes. Spontaneous emission can have a much broader radiation pattern, while stimulated emission tends to be directional. Light source 100 is designed to produce mainly spontaneous emission, and thus generate significant off-normal emission due to spontaneous emission, having an angular range of at least (≧) 30°. As described above, this is accomplished by designing the thickness of the active region 115 of the optical pump to be sufficiently large such that its lasing threshold is greater than the laser it is pumping. In this case the spontaneous emission stays below threshold, while using its spontaneous emission to optical pump the laser to above its threshold.

Furthermore it is desirable to make the spontaneous emission pump to have a threshold for lasing well above the maximum operating level desired for the optically pumped laser. In this way the spontaneous pump laser maintains its wide angular range for emission over the full range of operating conditions desired for the laser. By designing the light source 100 to be a spontaneous optical pump having a bulk thickness of at least 1 μm for the active region 115 a GaAs spontaneous pump, it can in principle maintain the active region 115 (e.g., a GaAs emitting region) in an absorbing condition (negative optical gain), it can be electrically driven to ˜4000 A/cm². In this way its threshold for lasing can be made well above that needed for optical pumping a laser into above threshold operation. Its operating range of current density can be further increased with increase the GaAs thickness. In some cases a 5 μm or more thick active region 115 providing a spontaneous emitting region may be desired, with could provide as much as 20 kA/cm² pump level to the lateral light source 440 operating as an optically pumped laser. In addition, active region 115 may be made thinner if the maximum drive level desired for laser operation is also for lower current density. The important aspect is that active region 115 remain in the spontaneous regime while pumping laser 440. The layers around active region 115 (i.e. the GaAs described above), can be made for efficient injection and doped p and n.

This off-normal emission can be effectively trapped between two mirrors, especially for angles beyond the critical angles of mirrors 110 and 120. Mirror 110 along with absorption by light source 100 combined with the drive level of light source 100, are designed to maintain light source 100 in a spontaneous regime so that pump emission by light source 100 covers a large angular range of 30 degrees or more. By using primarily spontaneous emission as opposed to stimulated emission from light source 100 the absorption of pump energy generated by light source 100 can be more effectively absorbed by the active region 441 of lateral light source 440. In this manner the efficiency of the composite light source 400 chip can be increased. Note that the angular emission shown in FIG. 4 also influences this lasing spot size. The angular emission may also be wider than that shown in FIG. 4, which will again affect the lasing spot size. This angular range will also be set by the optical pump's active region 115 thickness and absorption properties, and the degree of reabsorption of photons emitted by the optical pump maintained in the absorbing region. It is noted that maintaining the optical pump at a condition of optical transparency (zero gain) is also possible, but can make the efficiency sensitive to fluctuations in the operating conditions.

Spontaneous regime is maintained by choosing the light source 100 to have a thickness of its active region 115 so that active region 115 remains optically absorbing or near transparency to produce little stimulated emission, under the maximum desired pump level. For an example light source 100 shown in FIG. 1A having a substrate 105 comprising GaAs with a GaAs bulk thickness of 1 μm enables a pump level up to approximately 4000 A/cm². This GaAs thickness can be increased to as much as about 5 μm to 10 μm, enabling approximately 20,000 A/cm² of electrical drive to the GaAs light source while remaining absorbing or close to transparency. Therefore, high pump levels can be achieved with the high efficiency pump operating in the spontaneous regime.

Active region 441 may include one or more light emitting regions emitting at hν₂ or within some spectral range of this energy, and absorbing layers to absorb light at hν₁. By using the spontaneous emission from active region 115 to pump the active region 441, dopants can be removed the active region 441, and from the cladding layers surrounding active region 441. In this way heat generation that would otherwise come from electrons and holes and their free carrier absorption that accompany these dopants, or from free carrier absorption by electrons and holes that are otherwise injected and provide electrical transport from an external applied bias, can be reduced or eliminated, and optical loss due to absorption by free carriers can be reduced. By designing the layers of active region 441 to both provide a low threshold for lasing and sufficiently high potential barriers in the absorbing layers of active region 441 to keep electron and hole concentrations low in these absorbing regions, very low internal optical loss can be achieved in composite light source 300 for its own emission of light at photon energy hν₂.

In addition, the light source 100 may be doped heavily and be operated to produce a low drive voltage to the composite light source 300 of FIGS. 3A/3B by making the active region 115 a bulk emitter of high efficiency, and driving the active region 115 with a sufficiently low drive voltage the vertical cavity light source can be made to absorb heat in its active region 115. The amount of heat that can be absorbed depends on the thickness of active region 115, internal quantum efficiency, injection efficiency into the active region 115, and the bias voltage. In practice the active region 115 may be a few thousand angstroms in thickness to several microns. Active region 115 is therefore able to operate under some conditions as a heat pump, and absorb some its own heat from surrounding electrical contacting material and composite light source 300.

As noted above, lateral cavity optical loss can thus be reduced by having a low concentration (typically <5×10¹⁷ cm⁻³) of n doping in laser 440, including close to the active region 441 of the lateral light source 440, and having light source 100 driving it optically to excite lasing. Efficiency can be increased by using a sufficiently thick active region 115 that may range from 0.1 μm to 5 μm or more in thickness, to enable a low drive voltage across the p-n junction of the active region 115 of the light source 100. To further reduce the drive voltage and overcome charge transport limitations in the vertical spontaneous emitter, multiple emitting regions may be cascaded using tunnel junctions. By reducing electrical currents efficiency may be improved due to the larger fraction of voltage drop that occurs over the active region 115 of light source 100. Vertical pumping in composite light source 300 enables the drive voltage to be reduced to very small values including values less than the photon energy divided by the unit electron change q, or hν₂/q, and yet produce optical gain in the lateral cavity emitter if this lateral cavity emitter is a laser. Likewise it can produce gain if this lateral cavity emitter comprises a superluminescent diode.

Under the condition that the drive voltage of each separate spontaneous emitter region is less than its own photon energy hν₁/q, the light source 100 acting as a spontaneous emitter can be made to absorb heat generated by the laser. In this manner the spontaneous emitter not only serves to increase the efficiency of the laser by removing heat generating free carriers that would be associated with doping the laser, but can further increase the overall efficiency by converting some heat back into pump emission of the spontaneous emitter. By removing the free carriers and reducing the optical absorption, the lateral cavity laser can also be made to have a longer cavity without suffering significant optical absorption of its laser light, and thus its power output increased.

To increase the absorption efficiency of the active region 441 of the lateral light source 440 in absorbing the vertical emission provided by active region 115, a second mirror (not shown) can be placed between the spontaneous emitting active region 115 and the active region 441 to increase resonant absorption through field interference. In combination or separately, the active region 441 of the lateral light source 440 may be made to capture lateral spontaneous emission from active region 115 through coupled lateral waveguides. By maintaining the voltage bias of active region 115 to cause absorption of its own spontaneous emission, photon recycling can be generated to further increase the pump efficiency of the lateral cavity laser by the spontaneous emitting active region 115.

In addition, as noted above the upper DBR or other mirror 110 shown in FIGS. 1A and 1B may not be needed. If sufficient absorption is obtained in the active region 441 so that nearly all photons emitted from the spontaneous emitting active region 115 are absorbed in a single pass, the mirror 110 can be eliminated. Because the majority of the emission from light, in general, mirror 110 will reflect only a range of angles from the active region 115, so some light is always expected to pass. By properly choosing photon energies hν₁ and hν₂ such that hν₁>hν₂ by a few amounts of the thermal energy or more, nearly all light from the light source 100 can be absorbed in active region 441 if the absorbing layers are thicker than about 1.5 μm. For example the thermal energy is ˜26 meV at room temperature. In practice a thickness of 3 μm to 5 μm or more (e.g., 3 μm to 10 μm) may be used for the active region 441.

The low voltage contacting to the light source 100 shown in FIGS. 3A and 3B can be facilitated using the DHCBR 126(a) within the p-side mirror of light source 100. In this way high thermal conductivity and low drive voltage can be simultaneously obtained.

An important advantage of the composite light source 300 for the case that active region 441 forms a laser is that because the internal optical absorption of this laser can be reduced, its cavity length can be increased and greater power extracted from its aperture. Increasing this power from the aperture enables improved solid state and fiber laser sources.

For example, the composite light source 300 of FIGS. 3A/3B can be connected to an active or passive fiber of a given diameter, so that the semiconductor laser aperture coupled to the fiber is limited in its own aperture. Because the semiconductor laser can operate with increased cavity length because of its decreased internal optical absorption, it can also operate with an increased power. This enables an increase in power from the fiber coupled to the semiconductor laser. If the fiber is an active fiber to form a fiber amplifier or fiber laser, the fiber amplifier or laser will also be capable of greater power.

If the fiber is connected to a fiber network comprised of additional semiconductor lasers, with the fiber outputs optically coupled to a single fiber, the same power laser can thus operate with fewer semiconductor lasers and fiber couplings. This can reduce the size of the fiber laser system for a given power level. Because semiconductor lasers such as composite light source 300 shown in FIGS. 3A and 3B can be made to operate with lower internal electrical and optical losses, they can also be made to operate more efficiently. Therefore the improved fiber laser can also be made to consume less power for a given power output, and operate more efficiently.

Active region 441 may also be designed to operate in a superluminescent regime to maintain a broad spectrum. In this case the optical pumping by active region 115 can help to distribute the pump excitation among more than one emitting region placed in active region 441. In particular, by forming multiple quantum dot active layers that themselves exhibit little optical absorption for each other and emit at slight different center wavelengths, a very broad emission spectrum source can be constructed. This source can be useful to make an improved instrument for optical coherence tomography (OCT).

FIG. 4 shows an example composite light source 400 which integrates a light source 100 embodied as a spontaneous emitter with a vertical cavity light source comprising active region 441 that provides surface emission, according to an example embodiment. Composite light source 400 includes an n-electrode 121 just above the active region 115 and an p-electrode 111 on the bottom side of the lower mirror 120. Composite light source 400 is side (or edge) emitting.

FIG. 5 shows an example composite light source 500 that integrates light source 100 embodied as a spontaneous emitter having an active region 115 providing spontaneous emission formed between an n type region provide by lower DBR mirror 120 shown as a p-type mirror and n type region 530, with a second vertical cavity light source having its active region 525 shown that provides vertical surface emission, according to an example embodiment. The upper DBR 110 is shown as upper mirror 110 that is generally n-type.

The top electrode 121 is shown as an n-electrode that includes a center aperture to allow light emission therethrough as shown. The bottom electrode is shown as a p-electrode 111. Above the lower DBR mirror 120 is a disclosed DHCBR 126(a) having a p-type conducting channel 131. In this embodiment the second vertical cavity light source having the active region shown as 525 can be a VCSEL, which partially shares a cavity with the light source 100 having active region 115. By optically pumping the light source 100 (e.g., a VCSEL) with spontaneous emissions from the active region 115, as described above dopant impurities can be removed or reduced below 1×10¹³ cm⁻³ from the active region 525 to reduce free carrier absorption that causes both heating and optical loss.

In this embodiment the middle mirror 535 shown forms a reflector for the composite light source with active region 525, while effectively allowing light from the active region 115 of light source 100 to pass through the middle mirror 535 and be absorbed in active region 525. This middle mirror 535 can be designed to have two reflectivity bands by choosing its layer thicknesses based on multiples of the two center wavelengths. One of the reflectivity bands can be used to reflect the light from active region 525 with sufficient value to lase as a VCSEL, and a second reflectivity band that is used to cause resonant absorption of the spontaneous emission from active region 115. Likewise, the upper DBR 110 can also be made to have two reflectivity bands, one of which forms a VCSEL with active region 525, and one of which causes resonant absorption of the spontaneous emission from active region 115. The lower DBR 120 or other mirror can then be designed to have a reflectivity band only for the spontaneous emission from active region 115.

As with the case of the light source 100 shown in FIG. 1A, in the case of the composite light source 500 embodiment of FIG. 5 the deleterious effects of free carrier absorption causing heat generation and light absorption can be reduced by optical pumping, and heating due to electrical transport eliminated for the second VCSEL. In addition, multiple active regions can be added to the VCSEL that are resonant with its vertical lasing mode, so that its optical gain is increased and its cavity requirements for lasing reduced. Optical pumping can also produce more uniform lasing for large VCSEL areas. In addition, mesas can be added to the VCSEL with active region 525 and cavity extensions used to form phase coupling feedback, including Talbot feedback. Each of these arrangements can enable more desirable cavity conditions to be reached for high efficiency in lasing from their active region.

As described above, disclosed light sources include a robust form of a low cost laser or other light source by incorporating lithographic control of the optical mode that also produces polarization control, and forces current through an optimum electrical injection path. It forces the current through the injection path while also enabling easy heat flow through the device to reduce thermal resistance. Disclosed embodiments also enable low voltage operation and improved optical performance from semiconductor laser diodes.

Disclosed light sources have applications in visible light emitters, high speed optical interconnects, optical sensing, displays, and optical pumping. Sensor uses for disclosed embodiments include position sensors, gas sensors, bio-sensors, and time keeping (atomic clocks).

EXAMPLES

Disclosed embodiments are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of this Disclosure in any way.

FIG. 6 shows measured data for an example InGaAs quantum well p-n diode laser showing the current density vs. voltage, for a variety of p-conducting channel 131 diffused sizes ranging from 15 μm circular aperture to a 100 μm circular aperture. After an initial growth, the wafer was patterned lithographically and with selective etching to form mesas containing Be acceptor impurities intended for diffusion. After the wafer was returned to the epitaxial growth chamber following mesa fabrication, Be was diffused from the mesas in the growth chamber just prior to a second growth. The second growth was either a GaAs test structure to isolate the voltage-current characteristics of the DHCBR 126(a) and conducting channel 131, as used herein, or a mirror to complete an RCLED or a VCSEL. The test structure allows the isolation of the electrical properties of the DHCBR and p-conducting channel.

The DHCBR 126(a) included an 650 Å thick Al_(0.45)Ga_(0.55)As undoped layer clad on its lower side by p-type Al_(0.1)Ga_(0.9)As and on its upper side by p-type GaAs. Some undoped graded interfaces are also included to reduce the electrical resistance of the p-diffused conducting channel 131. The apertures include the DHCBR blocking region outside the p-channel diffused regions. The current vs. voltage characteristics show that the p-diffused heterostructure can reduce the voltage for a given operating current density by using current funneling into the conducting channel. The test laser included disclosed depleted heterojunctions for current confinement, with a selective p-diffusion (Be acceptors) to form the conducting channel 131.

The measured current density vs. voltage shown in FIG. 6 for the test structure includes a three quantum well InGaAs emitting region centered at 980 nm wavelength, the p-diffused region that shows that the voltage is 1.53 V for a current density of 10⁴ A/cm² for the 15 μm diameter device. When series resistance to the device is accounted for, the actual device voltage is 1.49 V for a current density of 10⁴ A/cm² in the 15 μm diameter mesa test device. The blocking voltage was 3.5 V for the outer current blocking region 132 that includes the DHCBR 126(a) for these devices.

A significant aspect of these measurements is that very small devices are expected to produce the smallest drive voltages for high excitation levels of the active region. This is because additional electrical resistances above and below the conducting channel 131 can be minimized by making small devices of ˜5 μm or less. A feature of this scaling can be seen in the current density vs. voltage curves of FIG. 6. As the device size reduces, the voltage required to create this current density decreases. This is because spreading resistance to the device active region more favorably decreases the voltage, and because the drive current for a given current density decreases. These small sizes can minimize spreading resistances, so that drive voltage is mainly due to the active region (e.g., p-n quantum well emitter) and conducting channel 131. The results shown in FIG. 6 evidence that the p-diffused conducting channel 131 can provide a very low drive voltage to the active region of three quantum wells. For a VCSEL the very low drive voltage can produce high efficiency as well as enable high speed modulation for optical data generation. Since the low drive voltage can be achieved in very small devices, it can also be used to make high efficiency displays using RCLEDs or VCSELs.

FIGS. 7A and 7B show spontaneous emission spectra and lasing spectra for the composite light source 300 shown in FIGS. 3A and 3B including the light source 100 shown in FIG. 1A which utilizes a vertical cavity spontaneous emitter to optically pump a lateral cavity laser having active region 441 embedded therein. More recent laboratory demonstrations have also used the DHCBR 126(a) to obtain electrical confinement in the lower mirror 120. For the data shown in FIG. 7, the composite light source 300 included active region 115 being an undoped 1.5 μm thick GaAs spontaneous emitting region between p and n Al_(0.1)Ga_(0.9)As layers. The p-layer included an Al_(0.7)Ga_(0.3)As reflector and Ag metal mirror and electrical contact. The light emission from the GaAs spontaneous emitting active region 115 is at wavelength ˜875 nm. The GaAs spontaneous emitter excites a GaAs region of the laser having active region 441 containing a InGaAs single quantum well, and emits at ˜990 nm. The GaAs region of the laser was 0.4 μm thick. The upper DBR mirror 110 was formed from 16 pairs of AlAs and Al_(0.1)Ga_(0.9)As layers, each having an optical quarter wave thickness relative to the 875 nm emission of the light source provided by active region 115. The AlAs layers in particular also produced large critical angles to increase the capture of wide angular emission from the GaAs spontaneous emitter by the laser having active region 441.

The laser cavity was 100 μm wide and 0.5 cm long, which is also the cross sectional area of the GaAs spontaneous emitter. In FIG. 7A the spontaneous emission that is composed of GaAs emission is shown at roughly 875 nm to 900 nm, and emission of the InGaAs quantum well that was optically pumped by the light source 100 being a GaAs spontaneous (QW) emitter at around 1,000 nm. FIG. 7B shows the emission at higher electrical driver levels to the active region 115 comprising a GaAs spontaneous active region, causing lasing in the InGaAs quantum well in region 441 at ˜990 nm.

While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the subject matter disclosed herein can be made in accordance with this Disclosure without departing from the spirit or scope of this Disclosure. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.

Thus, the breadth and scope of the subject matter provided in this Disclosure should not be limited by any of the above explicitly described embodiments. Rather, the scope of this Disclosure should be defined in accordance with the following claims and their equivalents.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including,” “includes,” “having,” “has,” “with,” or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 

The invention claimed is:
 1. A composite light source, comprising: a substrate having a top surface; a first vertical light source formed on said substrate, including: at least a lower mirror; a first active region above said lower mirror, said first active region having a thickness sufficient when electrically pumped to emit predominantly a spontaneous vertical emission from said first active region towards said top surface having an angular range of at least (≧) 30°; a second light source formed on said substrate above said first active region including a second active region, wherein said spontaneous vertical emission is at a first wavelength λ₁ that optically drives said second active region to provide an emission at a second wavelength λ₂, wherein λ₂>λ₁, further comprising a first electrode above said substrate and a second electrode on a bottom side of said lower mirror.
 2. The composite light source of claim 1, further comprising an upper mirror on said substrate above said second light source so that said first vertical light source is within a vertical resonant cavity.
 3. The composite light source of claim 2, wherein: said vertical resonant cavity includes an inner mode confinement region and an outer current blocking region; a depleted heterojunction current blocking region (DHCBR) within said outer current blocking region of at least one of said upper mirror, said lower mirror, and said first active region, and a conducting channel within said inner mode confinement region that physically contacts and is framed by said DHCBR, wherein said DHCBR forces current flow into said conducting channel during operation of said composite light source.
 4. The composite light source of claim 3, wherein a more lightly doped side of said DHCBR has a maximum p-type concentration of 1×10¹⁷ cm⁻³, and wherein said conducting channel has a minimum p-type concentration of at least 5×10¹⁷ cm⁻³.
 5. The composite light source of claim 3, wherein said conducting channel comprises a selective p-type diffusion.
 6. The composite light source of claim 3, wherein a more lightly doped side of said DHCBR has a maximum p-type concentration of 1×10¹⁶ cm⁻³, and wherein said conducting channel has a minimum p-type concentration of at least 5×10¹⁷ cm⁻³.
 7. The composite light source of claim 2, wherein said upper mirror comprises a p-type distributed Bragg reflector (DBR) and said lower mirror comprises an n-type DBR.
 8. The composite light source of claim 1, wherein said first vertical light source is a light emitting diode (LED).
 9. The composite light source of claim 1, wherein said second light source comprises a lateral light source.
 10. The composite light source of claim 1, wherein said first active region is between 1 μm and 5 μm thick.
 11. The composite light source of claim 1, wherein said second active region has a doping level <1×10¹³ cm⁻³.
 12. The composite light source of claim 1, wherein said second active region is undoped.
 13. The composite light source of claim 1, further comprising a first electrode above said first active region and a second electrode on a bottom side of said lower mirror.
 14. The composite light source of claim 1, further comprising a first electrode including a center aperture at a top of said composite light source and a second electrode on a bottom side of said lower mirror.
 15. The composite light source of claim 1, wherein said substrate comprises GaAs.
 16. A composite light source, comprising: a substrate having a top surface; a first vertical light source formed on said substrate, including: at least a lower mirror; a first active region above said lower mirror, said first active region having a thickness sufficient when electrically pumped to emit predominantly a spontaneous vertical emission from said first active region towards said top surface having an angular range of at least (≧) 30°; a second light source formed on said substrate above said first active region including a second active region, wherein said spontaneous vertical emission is at a first wavelength λ₁ that optically drives said second active region to provide an emission at a second wavelength λ₂, wherein λ₂>λ₁, further comprising an upper mirror on said substrate above said second light source so that said first vertical light source is within a vertical resonant cavity, wherein: said vertical resonant cavity includes an inner mode confinement region and an outer current blocking region; a depleted heterojunction current blocking region (DHCBR) within said outer current blocking region of at least one of said upper mirror, said lower mirror, and said first active region, and a conducting channel within said inner mode confinement region that physically contacts and is framed by said DHCBR, wherein said DHCBR forces current flow into said conducting channel during operation of said composite light source.
 17. The composite light source of claim 16, wherein said conducting channel comprises a selective p-type diffusion.
 18. The composite light source of claim 16, wherein said upper mirror comprises a p-type distributed Bragg reflector (DBR) and said lower mirror comprises an n-type DBR.
 19. A composite light source, comprising: a substrate having a top surface; a first vertical light source formed on said substrate, including: at least a lower mirror; a first active region above said lower mirror, said first active region having a thickness sufficient when electrically pumped to emit predominantly a spontaneous vertical emission from said first active region towards said top surface having an angular range of at least (≧) 30°; a second light source formed on said substrate above said first active region including a second active region, wherein said spontaneous vertical emission is at a first wavelength λ₁ that optically drives said second active region to provide an emission at a second wavelength λ₂, wherein λ₂>λ₁, further comprising an upper mirror on said substrate above said second light source so that said first vertical light source is within a vertical resonant cavity, wherein said upper mirror comprises a p-type distributed Bragg reflector (DBR) and said lower mirror comprises an n-type DBR. 